1. Field of the Invention
The present invention relates to the field of stencil printing, and more particularly to a technique of preparing a stencil master by perforating a thermal stencil sheet by a laser beam.
2. Description of the Prior Art
The laser is a technique developed in the 1950's as a technique of expanding the range of the microwave amplification by stimulated emission of radiation (maser) to the range of light wave frequencies, and from the beginning of its development it was known as a technique to be able to melt and cut a large variety of materials by irradiation of a light beam, as the technique makes it possible to generate a light beam having a high energy density. Therefore, as a matter of fundamental function and effect of the laser, it is readily thought of to produce a stencil master by irradiation by use of a laser beam to a heat-sensitive plastic film of a thermal stencil sheet so as thereby locally to perforate such a film.
However, since a laser beam is still a light beam, it passes through a transparent body, and therefore, when a laser beam is irradiated to a thermal stencil sheet made of a heat-sensitive plastic film having a relatively high transparency, most of the laser beam merely passes through the heat-sensitive plastic film. Therefore, in order to apply a heating effect such a thermal stencil sheet by a laser beam sufficient to cause perforation thereof, a laser beam is required to have such an extremely high energy density that the idea is, in fact, far from being applicable to such convenient small sized stencil printing devices for office use.
When an attempt is made to use a black stencil sheet such as disclosed in Japanese Patent Laid-open Publication 48-46417 (Patent No. 841178) filed by the same applicant as the assignee of the present invention) including fine particles of a light absorbing heat generating substance such as carbon distributed in a heat-sensitive plastic film to be perforated by a laser beam, it will be possible to perforate such a stencil sheet into a stencil master by a laser beam having a relatively low energy density. However, in order to produce a fine stencil print by using such a stencil sheet made of a heat-sensitive plastic film containing fine particles of a light absorbing heat generating substance, it is required that the fine particles of the light absorbing heat generating substance be distributed at high density and uniformity in the heat-sensitive plastic film. Nevertheless, since no chemical binding, which is generally a strong binding, is available between solid fine particles such as carbon particles and a heat-sensitive plastic, such fine solid particles are just held in the plastic layer only depending upon a mechanical planting. Therefore, when the density of the fine solid particles is increased, the fine solid particles are not sufficiently held in the plastic layer, and further the continuity of the plastic layer is so much damaged that a film having a uniform thickness is no longer available. Therefore, there is a definite limit in increasing the density of the light absorbing heat generating fine particles mixed in the stencil sheet.